(a) Field of the Invention
The present invention relates to a method and an apparatus for controlling shifting of an automatic transmission.
(b) Description of the Related Art
A typical automatic transmission includes a shift control apparatus that controls hydraulic pressure of solenoid valves, depending on various vehicle running states, such as vehicle speed and throttle valve opening, to shift gears.
The automatic transmission also includes a number of friction members. In each shifting operation, an off-going element is released, and an on-coming element is engaged. The operation of the off-going and on-coming elements is controlled by hydraulic pressure supplied thereto.
Deceleration lock-up control is sometimes employed in such a transmission. During deceleration lock-up control, a lock-up clutch is engaged at certain conditions. By such a lock-up clutch operation, a fuel-cut region of an engine is increased.
During coasting (i.e. a driver taking his foot off the accelerator pedal such that the vehicle nuns by its momentum), an engine control unit starts a fuel-cut control, and accordingly, engine speed decreases.
When the engine speed is decreased to a predetermined threshold speed, the engine control unit stops the fuel-cut control and restarts fuel supply to the engine such that the engine may produce power. Even if the engine power is not required, the engine is operated with the supply of fuel, which results in deterioration of fuel consumption.
In order to improve the fuel consumption, a deceleration lock-up control is employed in controlling the automatic transmission.
The lock-up clutch is controlled on and off even if shifting is not involved, and this repeated engagement and disengagement of the lock-up clutch may deteriorate the durability.
In addition, when a brake is operated while the lock-up clutch is engaged, the lock-up clutch is released in order to prevent engine stall and to improve noise-vibration-harshness (NVH) characteristics. This also reduces the fuel-cut region, and deteriorates the fuel consumption.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.